Like I said, I really cannot explain the error. It says the argument $arg is missing but the whole function is called only when that argument exists. So the whole error is completely unexpected, even on (apparently) a Windows server...

Unless there is something wrong with the WordPress installation. Are there any other weird errors occurring? Anything more meaningful in the server error log files? And what happens when you disable all other plugins and switch to the default theme?

We had to upgrade WordPress to the latest version and that solved all of the errors. We were wary of upgrading at first due to the designers of our template warning us that it could break with an upgrade to WordPress.

Hi JLeksell, thanks for coming back on this... I never paid attention to the fact you marked version 2.8.5 there. Still, it strikes me as odd but I'll mark the plugin as incompatible with only 2.9 and up.

I'm not going to test backward compatibility for those old versions anyway ;)